The operation of a completely mixed activated sludge pilot plant employing alum addition for phosphorus removal

Abstract

The public concern which has materialized in recent years regarding the quality of the nation's surface waters has created a need to upgrade wastewater treatment processes. New treatment objectives, in general, relate to the removal of phosphorus, achieving nitrification, and essentially eliminating the carbonaceous oxygen demand of wastewaters prior to discharge. The objective of this study was to operate a pilot plant scale activated sludge type treatment plant, using domestic sewage and employing alum as a coagulant in the primary process. Alum addition and flocculation were provided prior to a conventional primary clarification process. The secondary treatment consisted of a combination of the complete-mixed and plug-flow process operated in series.

It was found that this treatment scheme was capable of removing 95 percent of the total phosphate content of the municipal wastewater in contrast to the 20 to 30 percent removal normally provided by conventional activated sludge plants. The more efficient primary process gave an average reduction in phosphorus of 67 percent. The reduced carbonaceous and nutrient load to the secondary biological process provided more complete phosphate removal and resulted in a reduction in hard-to-handle secondary sludge. In addition to consistently high removals of phosphorus, the pilot plant achieved 93 percent nitrification and reduced influent BOD by 87 percent, on the average.

Chemical precipitation was found to be a stable and predictable means of upgrading the effluent of conventional secondary wastewater treatment plants. With a minimum of equipment and capital outlay, the chemical precipitation scheme could be adapted to existing plants or incorporated into the design of new plants.